The field of the invention relates to windshield wiper systems.
Conventional wiper systems couple an electric motor to the wiper blade or blades by a gearing mechanism. In one common approach, the direction of sweep across the windshield is reversed through a reversible linkage such that the motor rotates in only one direction. In another approach, blade sweep is reversed by reversing current through the motor. Both approaches are subject to a variety of problems. For example, blade velocity varies with blade wear, windshield conditions, battery voltage, and wear in the mechanical linkage. Further, when the blade abruptly reverses direction at the end of a sweep, the resulting blade flexing is both annoying to the vehicle operator and may impair complete removal of precipitation. There may also be variation between forward and reverse sweeps due to the mechanical characteristics in linkage between the motor and blade.
U.S. Pat. No. 4,663,575 issued to Juzswik et al on May 5, 1987 addresses some of these problems. This patent discloses an electric motor driving wiper blades via mechanical linkage. Forward and reverse blade sweep is accomplished by reversing current through the motor in response to electronic control circuitry. Before reversing directions, the motor is either accelerated or decelerated a predetermined time in response to a stored program or wiper profile. At best, however, this system can only correct for average blade velocity. More specifically, a cam signal provides an indication of when the blade has reached the end of a sweep. In response, the complete sweep duration is timed and compared to a desired sweep time. Current supplied to the motor is then adjusted such that the next sweep duration more accurately corresponds to the desired sweep duration.
The inventors herein have recognized numerous disadvantages to the prior approaches described above. For example, only the average blade velocity is adjusted. Stated another way, only the blade velocity averaged over a complete sweep is adjusted. Thus, when the blade traverses a slippery section on the windshield, it abruptly accelerates over that portion and then decelerates as shown in FIG. 1. The converse is true when the blade is impeded by a section of snow or slush on the windshield. These abrupt changes in velocity are both annoying to the operator and may impede complete removal of precipitation. The abrupt velocity changes may also result in blade chatter which reduces blade wear and is an additional annoyance to the operator. Further, since blade velocity is only corrected at the end of a sweep, a velocity change during a sweep may result in an unwanted pause between sweeps as illustrated in FIG. 1.